The present invention relates to alarm systems of the detecting loop type, for example, fire alarm circuits of the type classified by Underwriters Laboratories as Class B fire detection circuits.
Single detecting loop alarm circuitry conventionally utilizes a detecting loop from the central station to remote premises; the loop carries a dc current provided by a power supply at the central station. A resistor in parallel with a normally open detection switch, such as a heat sensitive switch, is provided in series with the loop at the remote premises. The loop lead which returns to the central station is coupled to an alarm circuit which utilizes two current-sensitive relays in series, a first relay which switches upon an increase in current, such as caused by closing of the detection switch and known as an "alarm" condition, and a second relay which switches upon a decrease in current, such as caused by a ground or open of the detecting loop and known as a "trouble" condition.